Color television receiver using switched synchronous demodulator



May 7, 1968 COLOR TELEVISION D. M. SANDLER 3,382,317

RECEIVER USING SWITOHED SYNCHRONOUS DEMODULATOR Original Filed Oct. 15,1964 2 Sheets-Sheet 1 COMPATIBLE F l I 'ffiQ/SIDN 876$ VSBF l4 g R Y 222e '5 RED 3 DELAY SNYC) ADDER TRANS.

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ATTORNEG United States Patent 3,382,317 COLOR TELEVISYLON RECEIVER USINGSWITCHED SYNCHRONOUS DEMODULATOR Donald M. Sandler, Oak Park, Ill.,assignor to Polaroid Corporation, Cambridge, Mass., a corporation ofDelaware Original application Oct. 15, 1964, Ser. No. 404,047. Dividedand this application Oct. 20, 1965, Ser. No. 498,887

1 Claim. (Cl. 178-54) This application is a division of copending parentapplication Ser. No. 404,047 filed Oct. 15, 1964.

This invention relates to decoders for recovering color information fromcolor television signals transmitted in accordance with the technicalstandards established by the Federal Communications Commission in 1953,and more particularly to a decoder designed to recover from suchsignals, sufficient color information for a one-gun kinescope to beoperated according to the red-white system of color analysis to producea color picture of the scene being televised.

The red-white system of color analysis requires the relatively longwavelength (i.e., red) content of a scene to be displayed in reddishcolored light and the relatively short wavelength (i.e., green) contentof the scene to be displayed in achromatic light. A one-gun kinescopethat operates according to this system of color analysis to produce acolor picture of a scene being televised is disclosed in copendingapplication Ser. No. 297,341, filed July 24, 1963, now Patent No.3,290,434 owned by the assignee of this application. Briefly, suchkinescope includes a target screen whose covering comprises twosuperposed cathodoluminescent layers that emit red and cyan lightrespectively when excited by a beam of electrons. When the acceleratingvoltage of the kinescope is adjusted to provide a beam with a relativelylow level of energy, electrons can penetrate only into the red layer,which is closer to the gun, causing the color of the light on the screento be red; and when the voltage is adjusted to provide a beam with aparticular relatively higher level, electrons can penetrate into thecyan layer simultaneously exciting both layers to produce red and cyanlight in such relative proportions that the light on the screen appearsto be achromatic.

To reproduce a scene in color with a one-gun kinescope such as this, itis necesary to apply the red video signal to the gun when theaccelerating voltage is adjusted to provide the lower energy beam; andto apply the green video signal to the gun when the accelerating voltageis adjusted to provide the higher energy beam. This involves asequential switching of the video signal applied to the gun between thered and green in synchronism with the sequential switching of theadjustment to the accelerating voltage. When the switching occurs at thefield frequency the result is a field sequential color system in which acolor frame corresponds to a scanning frame. In view of the fact thatonly red and green video signals are necessary to the red-white system,and, with a one-gun kinescope, that such video signals are requiredsequentially, it is readily apparent that the standard color televisionsignal, from which information on the red, green and blue content ofeach-picture element can be obtained simultaneously, will provide morecolor information than is actually needed. This suggests thatconventional decoding processes, such as those associated with tri-colorkinescopes and which can recover the simultaneous red, green and bluesignals contained in a standard color signal, can be coupled with aproperly synchronized gating circuit in order to sequentially apply theproper one of either the red or green video signals to the kinescopegun. While satisfactory operation will be achieved, this approach failsto take advantage of particular properties of both the standardtransmitted signal and the red-white system of color analysis whichpermit a much simpler decoder to be utilized. It is the provision of adecoder of this type that constitutes the primary object of the presentinvention.

Before briefly describing the present invention, the components of atransmitted color television signal that meets the technical standardsof the FCC will be reviewed in order to provide an antecedent basis forthe terminology used in this disclosure. Basically, the standardbroadcast color signal has a portion which conveys only brightnessinformation and is independent of the color of the scene beingtelevised; and a portion which conveys only color information and isindependent of the brightness of the scene. Brightness information issupplied by a luminance or Y signal matrixed from the video signalsderived from the synchronized and simultaneous scanning of red, greenand blue color-separation images of the scene in proportion to thecontribution of each color to brightness. The Y signal has a 4 me.bandwidth and will produce a high quality monochrome picture on theviewing screen of a tri-color receiver. Color is added to thismonochrome picture by the I and Q signals which constitute the colorinformation portion of the broadcast signal. The bandwidths of the I andQ signals and the precise manner in which they are matrixed from theprimary color video signals, provide the monochrome picture with only somuch of the red, green and blue content of each picture element as isnecessary for the reproduced scene as a whole to be interpreted by anaverage observer as being in full color. In large colored areas of thescene, which produce video frequencies less than 0.5 mc., the averageobserver easily resolves all three primary colors; and all three signalsmust be present in this frequency range to permit the large areas to bereproduced in full three-color fidelity with a tri-color kinescope. Inmedium sized colored areas which produce video frequencies between 0.5and 1.5 mc., the average observer delineates most sharply betweenorange-red and blue-green; so that only two signals need be present inthis frequency range to permit this special two-color reproduction ofmedium sized areas. Accordingly, the Q signal is the narrow bandchrominance component and is bandlimited to 0.5 mc.; while the I signalis the wideband chrominance component whose frequency extends to 1.5 me.and is matrixed from the primary color video signals to provide colorinformation along the orange-red to blue-green axis of the chromaticitydiagram. For fine colored detail, which produce video frequenciesexceeding 1.5 mc., resolution is by way of variation in brightness, sothat only the Y signal need be present.

Having established the minimum information which is required forreasonable reproduction of the scene in color, the three signals are socoded that the broadcast color television signal simultaneouslytransmits the luminance (brightness), dominant wavelength (hue), andpurity (freedom from dilution by white light) coordinates of the scannedpicture element. The I and Q signals provide the latter coordinatesbecause of the manner in which the FCC signal is broadcast. Thebroadcast signal is required to correspond to a luminance component (theY signal) transmitted as amplitude modulation of the main picturecarrier of the television channel and a simultaneous pair of chrominancecomponents (the I and Q signals) transmitted as the amplitude modulationsidebands of a pair of suppressed subcarriers in phase quadrature havingthe common frequency relative to the picture carrier of 3.58 me. Todevelop this required color signal, the total video voltage which ismodulated on the main picture carrier (not including sync and blankinginformation) includes the Y signal, already described, and a chrominancesignal obtained by generating a 3.58 me. color subcarrier, splitting itinto subcarrier components that are in phase quadrature and modulatingeach of the I and Q signals on a different one of the subcarriercomponents using a suppressed subcarrier modulation technique. Theresultant signal, containing only the sidebands of the two quadraturesubcarrier components, is termed the coded chrominance signal. It is inthe form of a 3.58 mc. subcarrier whose amplitude, when a given pictureelement is being scanned, is a measure of the product of the luminanceand purity of the element, and whose phase is a measure of the dominantwavelength of the scanned element. Roughly, it can be considered thatthe amplitude of the chrominance signal determines the saturation of thecolor to be reproduced and the phase determines the dominant wavelength.

Since suppressed subcarrier transmission is involved, recovery of theintelligence contained in the chrominance signal involves a synchronousdemodulation process which requires, in one standard approach todecoding, creating at the decoder, two 358 mc. subcarriers in phasequadrature. The latter may be developed by splitting the output of asuitable stable local oscillator of the required frequency intoquadrature components to define a pair of decoder subcarriers; but phaseinformation must be available if the phases of the latter are to berelated to the phases of the two subcarrier components of the colorsubcarrier. To provide such information, a burst of the color subcarrieris gated onto the back porch interval of the horizontal blanking pulseswhich are generated at the transmitter for line sync purposes. The burstis used at a receiver in an automatic phase control loop to referencethe phase of the output of the local oscillator to the phase of theburst.

As indicated above, the video voltage E used to modulate the mainpicture carrier is the sum of the Y signal and the chrominance signal EThe latter can be expressed mathematically as follows:

R=video signal dependent on red content of picture element G=videosignal dependent on green content of picture element B=video signaldependent on blue content of picture element RY=red color differencesignal which is a measure of the red content of a picture element lessall brightness information B--Y=blue color difference signal 40: angularfrequency of the color subcarrier, radians per second t=time, seconds.

The phase reference in Eq. 1 is the phase of the burst plus 180. Sincethe I phasor leads the Q phasor by 90, it follows that the phase of thecolor subcarrier (burst) must lag the I and Q signals by 303 (l80+l23)and 213 (l 80+33) respectively. This is accomplished in practice bycausing the phases of the components of the color subcarrier on whichthe I and Q signals are modulated to lag the burst by 57 and 147respectively.

4 Below 500 kc., which is the only frequency range in which the Y, I andQ signals can exist simultaneously, the chrominance signal has thefollowing form:

As can be seen by inspection of Eq. 6, the coded chrominance signal isindeed in the form of a 3.58 mc. subcarrier modulated in both amplitudeand phase .according .to the color content of the picture elements. Thephase of the coded chrominance signal Eq. 8, is an angle whose tangentis proportional to the ratio of two color difference signals, so thatthe angle associated with a scanned picture element is independent ofthe saturation of the primary color components of such picture element,and dependent only on the dominant wavelength of the light emanatingfrom the element. It will be recalled that the dominant wavelength ofcolored light is the wavelength of homogeneous spectral light that mustbe mixed with achromatic light to achieve a visual match with thecolored light. The variation of phase of a coded chrominance signal fora number of important colors is indicated in Chart A:

CHART A Normalized chrominance primary color phasor leads Color videosignal Y (,5, degrees burst by,

"' degrees Saturated red 1 0 0 0.3 103.5 283.5 Saturated green 0 1 0 0.59 240.7 00.7 Saturated blue"-.- 0 0 1 0.11 347 167 Saturated cyan... 01 1 0.70 283.5 103.5

The synchronous demodulation of a coded chrominance signal of the formshown in Eq. 6 with a reference signal at the color subcarrier frequencyand a phase is termed synchronous demodulation at The output of asynchronous demodulator is proportional to the product of the magnitudeof the coded chrominance signal and the cosine of the angle between thephasor representing the chrominance signal and the phasor representingthe reference signal. If the signal obtained as a result of synchronousdemodulation at is termed e then:

x=l cl COS (-x) where the angle is a function of the dominant wavelengthof the scanned picture element as already indicated.

From the definitions of Eqs. 6 and 7 and trigonometric identities, Eq. 9reduces to:

e sin ra t- 5 cos x (1O) where r=0.63 sin (o -13.5") (l2) g=0.59 sin+29) b=0.45 sin p 77) By simultaneous synchronous demodulation ofseparate portions of the coded chrominance signal with a pair of decodersubcarriers at different angles, two detected signals are simultaneouslydeveloped, each of which provides a single equation in three unknowns,namely, R, G and B which represent the red, green and blue luminances inthe scanned picture element. The third simultaneous equation in the samethree unknowns is the Y signal. Matrixing of the two detected signalswith the Y signal (simultaneous solution) actually provides thequantities R, G and B which can be applied to the red, green and blueguns of a tui-color kinescope. It should be noted that synchronousdemodulation at 123 and 33 provides the I and Q signals respectively,and this process is sometimes referred to as demodulation along the Iand Q axes. The equations in the three unknown lumin'ances resultingfrom synchronous demodulation along significant axes, and obtained fromEq. 8, are listed in chart B:

To appreciate the invention which is the subject matter of the presentapplication, it should be recalled that a one-gun kinescope requires thesequential presentation to the gun of the decoder red video and thedecoder green video. In a simultaneous transmission and decoding systemwhich uses a single synchronous demodulator to produce the decoder redand green signals simultaneously, it is conventional to employ some typeof switching arrangement by which one at a time of the two availablevideo signals is sequentially applied to the gun. This invention, then,involves a recognition that the two desired and unadulterated videosignals can be obtained sequentially from a coded chrominance signalcontaining simultaneous information on the red, green and blue contentof a scanned picture element by synchronously demodulating the codedchrominance signal, sequentially, at predetenmined angles that producethe desired un adulterated video signals. With this approach, thedemodulator itself performs the switching function thus reducing thedecoder complexity while at the same time permitting recovery ofunadulterated red and green video signals. For example, synchronousdemodulation can be carried out at the field frequency along the RY axisand the G-Y axis to obtain the red and green color difference signals byshifting the phase of the decoder subcarrier relative to the burst from90 to 303.2 (see Chart A) with a square wave synchronized with thevertical sync pulses. Dematrixing to obtain the red and green signalscan then be accomplished using a keyed matrixamp-lifier to provide theproper channel gain associated with demodulation on these two axes, andusing the kinescope to add the inverted luminance signal to the twocolor diiference signals as they are applied sequentially to the grid ofthe kinescope. Since the net voltage controlling the beam is thegrid-to-cathode voltage, the kinescope itself adds the luminance signalto each of the color difference signals permitting the unadulterated redand green video signals to be applied, sequentially, to excitation ofthe cathodoluminescent elements of the target.

One of the broadest views of the present invention is essentially that asequential pair of video signals, individually representative of twodifferent color characteristics of the scene being televised, can berecovered from the FCC approved signal by sequentially synchronouslydemodulating the chrominance signal at two different angles selectedsuch that keyed, individual matrixing of the sequential pair ofdemodulated signals with the luminance signal provides the sought aftersequential pair of video signals.

The more important features of this invention have thus been outlinedrather broadly in order that the detailed description thereof thatfollows may be better understood, and in order that the contribution tothe art may be better appreciated. There are, of course, addi tionalfeatures of the invention that will be described hereinafter and whichwill also form the subject of the claim appended hereto. Those skilledin the art will appreciate that the conception upon which thisdisclosure is based may readily be utilized as a basis for designingother structures for carrying out the several purposes of thisinvention. It is important, therefore, that the claims to be grantedherein shall be of sufficient breadth to prevent the appropriation ofthis invention by those skilled in the art.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

FIGURE 1 is a block diagram of transmitter apparatus for coding the red,green and blue video signals individually associated with the scans ofthe three colorseparation images according to FCC regulations to produceand transmit a composite video signal made up of luminance signal and aphase and amplitude modulated chrominance signal;

FIG. 2(a) is a phase diagram of the coded chrominance signal showing theamplitude and phase of the chrominance signal that results from the scanof a particular picture element;

FIG. 2(b) is a phase diagram of the coded chrominance signal showing theresults obtained by synchronous demodulation of the signal at threedifferent angles;

FIG. 3 shows a matrix arrangement for generating from the demodulatedchrominance signal a pair of decoder red and green video signalssuitable for use by a red-white color kinescope;

FIG. 4 is a block diagram of a television receiver constructed inaccordance with the present invention showing a simplified decoder inwhich the demodulator functions as a switch for controlling thesequential application of the video signals to the one gun of thekinescope;

FIG. 5 is a synchronization diagram showing how color switching isobtained and the relationship between the color of the video and thecolor of the screen;

FIG. 6 is typical of the response characteristic of a modulated phaseshifter; and

FIG. 7 is typical of the response characteristic of a voltage-sensitivevariable attenuator which is part of a keyed matrix-amplifier.

Referring now to FIGURE 1, reference numeral 10 designates equipment forgenerating and transmitting color television signals in accordance withthe technical standards established by the Federal CommunicationsCommission. Such equipment forms no part of the present invention, beingentirely conventional, and is included in block diagram fonm forreference purposes only. Equipment 10 comprises direct pick-up camera11, signal processing equipment 12, color coder 13 and radio transmitter14. Apparatus for developing the four basic timing signals: horizontaldrive, vertical drive, blanking and sync are not shown, it beingunderstood that such apparatus develops the timing signals from a masteroscillator (not shown) stabilized to produce a 3.579545 mc.continuous-wave signal (the nominal 3.58 mc. color subcarrier) Camera 11contains a light splitting optical system 15 for the purpose ofpresenting a red color-separation image of the scene being televised tothe sentitive surface of pick-up tube 16, termed the red pick-up tube; agreen color-separation image to pick-up tube 17, termed the greenpick-up tube; and a blue color-separation image to pick-up tube 18,termed the blue pick-up tube. The preamplifiers and the horizontalandvertical-scanning generators normally associated with each color channelof camera 11 have been omitted to simplify the drawing, it beingunderstood that each channel is provided with adjustments so as to haveidentical characteristics. As is conventional, the drive pulses appliedto each of tubes 16, 17 and 18 cause the scanning beam of each to bedeflected in synchronism according to the standard odd-line interlacedscanning program; and the resultant outputs of the three tubes areapplied to signal processing equipment 12 in order to accomplish gammacorrection, aperture control, shading correction and pedestal insertion.As a consequence of this, there are three primary color outputs fromequipment 12 labeled in the drawing R, G and B. Output R, associatedwith red pick-up tube 16 produces a signal proportional to the redcontent of the scanned picture elements, and is termed the red videosignal; output G, associated with green pick-up tube 17 produces asignal proportional to the green content of the scanned pictureelements, and is termed the green video signal; and output B, associatedwith blue pick-up tube 18, is termed the blue video signal. Since thescanning of the photosensitive areas of the pick-up tubes issynchronized (with the three tubes in registration to provide rastershaving identical sizes, shapes and positions relative to the scene beingtelevised), the same picture element of each colorseparation image isscanned simultaneously. Thus, at any instant, each video signal isrepresentative of the bright ness of a different one of the primarycolors contained in the same picture element.

After gamma and other necessary corrections, the R, G and B camerasignals are applied to color coder 13 in order to adapt them forcompatible transmission with the existing 6 me. television channel. Thecolor coder accomplishes its function by the use of matrixing,suppressedcarrier modulation, quadrature modulation, VSB transmission ofthe I signal, and bandwidth limitation of the Q signal, together withvarious incidental gating, adding and subtracting operations, all ofwhich are conventional. Matrix circuit 19 linearly combines the R, G andB signals in accordance with Eqs. 2, 3 and 4 to define the 1,, and Qsignals and the Y signal respectively, where the subscripts on thechrominance components indicate that they are wideband at this point inthe circuit. Filters 20 and 21 limit the video frequency chrominancesignals to 1.5 me. for I and 0.5 me. for Q; and delay lines 22 and 23 inthe wider band channels equalize the time passage for all signalcomponents. The 3.58 me. color subcarrier is delayed 57 and split intotwo components, one of which is modulated at 24 with the I signal andthe other of which is delayed by 90 and modulated at 25 with the Qsignal. Modulators 24 and 25 are doubly balanced to produce only thesideband frequency components. Finally, the luminance signal Y, thequadrature sidebands outputs of modulators 24 and 25, deflection-syncsignals and the color burst are all summed in adder 26 whose output istermed the composite video signal and constitutes the total videovoltage modulated on the main picture carrier to produce the broadcastcompatible color television signal previously described in detail.

As indicated previously, the composite video signal modulated on themain picture carrier (less sync information) comprises the Y signal andthe chrominance signal, the latter being in the form of a 3.58 rnc.subcarrier whose phase, when a given picture element is scanned, isdetermined by the dominant wavelength of the scanned element, and whoseamplitude is determined by the saturation of the color. A phase diagramfor the coded chrominance signal is shown in FIG. 2(a) to whichreference is now made. The color subcarrier component on which the Isignal is modulated lags the color subcarrier (burst) by 57; and thecolor subcarrier component on which the Q signal is modulated lags thecolor subcarrier by 147. FIG. 2(a) shows the instantaneous I and Qphasors resulting from the scanning of a picture element having somearbitrary combination of the three primary colors. The instantaneous Ephasor in FIG. 2(a), ob-

tained by the vector addition of the I and Q phasors, indicates that thescanned picture element corresponding to this E phasor must bebluish-red since the first quadrant of the chrominance phase diagramdefines colors that lie in the range red to magenta to blue. The secondquadrant defines colors in the range red to orange to yellow; the thirdquadrant defines colors in the range yellowish-green to green tobluish-green; and the fourth quadrant defines colors in the range cyanto blue.

The result of synchronous demodulation of the coded chrominance signalshown in FIG. 2(a) along three representative axes is shown in FIG.2(b). Synchronous dcmodulation, it will be recalled, provides ademodulated signal whose amplitude is proportional to the product of themagnitude of the coded chrominance signal and the cosine of the anglebetween the latter signal and the decoder subcarrier reference signal.In other words, synchronous demodulation provides the projection of thecoded chrominance signal on the axis identified with the phase of thedecoder subcarrier reference signal. Chart B lists the color content ofthe demodulated signal for the axes shown in FIG. 2(1)); and Chart Alists the phase of the decoder subcarrier reference signal necessary todemodulate along the axes listed.

Apparatus by which the technique of obtaining essentially red and greendecoder video signals by synchronous demodulation along preselected axesfollowed by matrixing with the luminance signal, can be applied to aone-gun kinescope that operates on the red-white principle of coloranalysis, is shown in FIG. 4 to which reference is now made. Referencenumeral 30 designates a receiver into which such apparatus isincorporated and includes bicolor kinescope 31, decoder 32 and receivercircuitry 33. As indicated in copending application Ser. No. 297,341referred to above. kinescope 31 includes at one end, a viewing screen 34having a covering 35 thereon that constitutes a target for a beam ofelectrons produced by single electron gun 36 at the other end of thekinescope. Covering 35 can be constituted by two superposed granularcathodoluminescent layers which emit red and minus-red (cyan) lightrespectively, with the red light emitting layer being closer to the gunand being uniformly distributed over but covering less than 100% of theviewing screen. Additionally, a nonluminescent barrier layer separatesthe two cathodoluminescent layers. With this construction, about a 10kv. accelerating (target) voltage is suflicient to excite only the redlight emitting layer, with interstitial electrons of this energy thatpass between the granules being stopped short of the minus-red layer bythe barrier layer; and red light is produced on the screen. At higheraccelerating voltages, however, interstitial electrons have suflicientenergy to pass through the barrier layer and penetrate the minus-redlayer whereby both the red and minus-red layers are simultaneouslyexcited; and particularly, at about 20 kv., both layers will besimultaneously excited into emission of substantially the same amount oflight whereby achromatic light is produced on the screen. Thus,red-white color switching can be achieved by modulating the acceleratingvoltage between 10 kv. and 20 kv.

In operation, a broadcast color television signal received at antenna 37is converted to IF by tuner 38, amplified and then detected at 39 toproduce the composite color signal that existed at the input to thetransmitter at the sending end of the television system. Syncinformation is separated at 40 from the composite color signal andapplied to the horizontal and vertical deflection generators whichproduce sawtooth current pules that are applied to the horizontal andvertical windings of deflection coil 41 of the kinescope to cause thebeam to trace out the conventional raster in the terms of sequentialinterlaced fields. Associated with the horizontal deflection generatoris high voltage power supply 42 which provides the 10 and 20 kv.potentials that must be applied sequentially to covering 35 by means ofelectronic switch 43, the action of which is synchronized with thescanning of each field of a frame by the output of square wave generator44 producing a square wave at the field frequency and synchronized withthe vertical pulses derived from the vertical deflection circuit. As aresult of this arrangement, the voltage on covering 35 remains at kv.during one field scan of the covering by the beam to produce a redfield, and at kv. during the next field scan to produce a white fieldinterleaved with the red field.

Modulation of the target voltage at the field frequency normally resultsin the red field being larger than the white field with the result thatimages reproduced during successive fields will not be in registrationunless compensation is provided. To this end, it is conventional toprovide an electron permeable mesh designated by reference numeral 45between the gun and covering 35, and as close to the latter as possiblebut electrically insulated from the covering. As indicated in copendingapplication Ser. No. 344,914, filed Feb. 14, 1964, and owned by theassignee of the present application, misregistration between the twofields can be reduced to a minimum by applying a voltage to mesh 45 thatis modulated in synchronism but 180 out-of-phase with modulation of thetarget voltage. In par ticular, good registration is achieved inkinescopes where the target voltage is modulated between 10 and 20 kv.when the mesh voltage is modulated about an average voltage of 12.5 kv.with a peak-to-peak signal of about 1600 volts. These desired voltagescan be obtained from power supply 42 and applied to mesh 45 by way ofelectronic switch 46, the action of which is synchronized with thescanning of each field by the output of square wave generator 44. Thesequence and phase relationships between the target and mesh voltagesare indicated in FIG. 5, which also indicates that reproduction of thescene in color requires a video signal based on the green content of thescene being televised to be applied to the gun during the time intervalsthat the target voltage is maintained at 20 kv., and on the red contentduring the time intervals that the target voltage is maintained at 10kv.

It is the function of decoder 32 to provide video signals based on thered and green contents of the scene being televised and to sequentiallypresent such signals to the gun of the kinescope properly synchronizedwith the color switching operation. In operation, the composite colorsignal at the output of detector 39 is amplified at 47 and in aconventional manner, the chrominance and luminance components of thecomposite color signal are separated. The chrominance signal is selectedby the chromin-ance amplifier, which, typically, may include a bandpasstakeoff filter, the amplifying tube, a shaping filter plus trappingagainst 4.5 mc., all indicated schematically at 49. The selectedchrominance signal is applied via the burst take-off connection to agated amplifier (not shown) which, under control of the horizontalflyback pulses, transmits color bursts to the APC (automatic phasecontrol) circuit 50. This is a conventional approach to providing anerror signal to control the decoder subcarrier phase and forms no partof the present invention.

As previously indicated, the function of decoder 32 is to sequentiallysynchronously demodulate the chrominance signal along the positive (R-Y)axis and the positive (GY) axis at the field frequency. Along thepositive (R-Y) axis, the demodulated signal is:

90=0.877(RY) which means that the unadulterated red video signal associated with the red pick-up tube of the cameras can be obtained usingthe kinescope as the matrix if the red color difference signal isobtained as follows:

On the other hand, along the positive (GY) axis, the demodulated signalis:

1 0 which means the unadulterated green video signal associated with thegreen pick-up tube of the camera can be obtained using the kinescope asthe matrix, if the green color difference signal is obtained as follows:

Decoder 32 provides a simple device for the recovery of unadulteratedcolor signals. Decoder 32 requires no video switch because the propercolor difference signal is automatically applied to the grid of thekinescope as a result of switching the angle at which demodulationoccurs; and multiple matrices are eliminated by the use of a keyedmatrix-amplifier whose operation is synchronized with the angle at whichdemodulation occurs to provide the matrixing operation necessary toachieve the desired color-difference signal. The total matrixingoperation, including that achieved using the grid of the kinescope aswell as the matrix amplifier, is schematically illustrated in FIG. 3where the operations, carried out sequentially, are as follows:

In FIG. 4, the phase shifter shown at 61 is provided with means forvarying the angular shift of the output of the 3.58 mc. oscillator shownat 62 to accomplish sequential demodulation at different phase angles.Assuming that the phase of the output of oscillator 62 lags by the phaseof the burst, it is evident from chart B that phase shifter 61 must varythe phase of the decoder subcarrier reference signal applied tosynchronous demodulator 52 between 0 and 213.2 (which is the same as 90and 303.2 respectively relative to the burst) at the field frequency.This can be accomplished by the use of a voltagevariable capacitorelement in the phase shifter, as for example, a reactance tube.Preferably, however, a variable capacitor diode is used. In eitherevent, the phase shifter 61 will have some characteristic that relatesphase shift to the voltage applied to the variable capacitor element ina manner suggested by wave 64 in FIG. 6. This establishes the amplitudeof the square wave indicated by curve 65 that must be applied to thevariable capacitor element, and it is the function of amplifier 66 tofurnish the required signal. Amplifier 66 is driven by square wavegenerator 44 so that the phase shifter sequentially varies the phase ofthe decoder subcarrier reference signal between 0 and -213.2 relative tothe output of the subcarrier oscillator at the field frequency.

Demodulator 52 and keyed matrix-amplifier 63 represent ademodulator-matrix channel whose characteristic has a value dependingupon circuit parameters. For example, if the input to decoder 32 were KE where K is a constant defining the voltage level after detection, thedemodulator-matrix must have characteristics of 1.14 K/K to provide anoutput of K(RY) when the local or decoder subcarrier produced byoscillator 62 has an angle of 90 relative to the phase of the burst, and0.71 K/K to provide an output of K(G Y) when the angle is 303.2. Thevalue K is a constant of proportionality. In other words, thecharacteristic of the channel must decrease by a factor of about 38%when the phase of the reference signal changes from 0 to 213.2. This canbe accomplished, for example, by using conventional diode waveshapingtechniques whereby the application of a square wave signal to a diode ofa voltage-sensitive variable attenuator circuit would shift theattenuation of the output of demodulator 52 by the required amount insynchronism with the shifting of the angle at which demodulation occurs.The characteristic of the keyed matrix-amplifier, of which thevoltage-sensitive variable attenuator is a portion, may have the shapeshown at 67 in FIG. 7. The break in the curve 67 results from a changein the state of conduction of the diode of the variable attenuator, andestablishes the amplitude of the square wave indicated by curve 68 thatmust be applied to variable attenuator circuit; and it is the functionof amplifier 69, driven by generator 44, to furnish the required signal.

The sequence of operations of the receiver shown in FIG. 4 is controlledby square wave generator 44. When the output of the latter causes targetswitch 43 to apply kv. to the target and mesh switch 46 to apply 11.7kv. to the mesh, the output of amplifier 66 causes the capacitanceassociated with phase shifter 61 to have a value which places the phaseof the decoder subcarrier reference signal applied to demodulator 52 at-303.2 relative to the burst (or 2l3.2 relative to the output ofoscillator 62). The output of demodulator 52 under this condition is K eu which, as was shown previously, is 1.42 K (G-Y). At the same time, theoutput of amplifier 69 applied to matrix-amplifier 63 causes thevariable attenuator to have a value which results in the output ofdemodulator 52 (K e o) being multiplied by the factor 0.71 K/K Thisyields K(GY) at the grid of the gun of kinescope 31. The output of theluminance amplifier is -KY and this is applied to the cathode of thegun. Since the resultant signal on the gun is the difference between thegrid and cathode voltage, the video signal controlling the intensity ofthe beam during the time a field in achromatic light is traced on theviewing screen is KG, a signal proportional to that associated with thegreen pick-up tube at the camera. When the vertical sync pulse switchesthe output of the square Wave generator such that 10 kv. is switchedonto the target by switch 43 and 13.3 kv. is switched onto the mesh byswitch 46 for the next field scan, the output of amplifier 66 causesdemodulation to switch from 90 to 236.8 (or from -90 to 303.2respectively, relative to the burst); and the output of amplifier 69causes the matrixamplifier to multiply the output of demodulator 42, nowKlego by the factor 1.14 K/K This yields the signal K(RY) at the grid ofthe gun. The kinescope again performs matrixing with the Y signal. Thus,during the time a field in red light is traced on the viewing screen,the video signal KR proportional to that associated with the red pick-uptube at the camera, controls the intensity of the beam. As alreadyindicated, this is the requirement for reproducing a scene in full colorusing the red-white system of color analysis.

The approach to decoding using the kinescope to perform a portion of thematrixing achieves the same end results as when all of the matrixing iscarried out without the use of the kinescope. For this reason, the termmatrixing as used in the claims is intended to cover both approaches.

While the preferred embodiment just described provides a keyedmatrix-amplifier at the output of the synchronous demodulator, thoseskilled in the art will appreciate that the variable attenuator portionof the matrix-amplifier could be inserted, instead, in either input tothe demodulator.

The embodiment of the invention described above, while particularly welladapted for reproducing a scene in color using the red-white system ofcolor analysis, can be used to advantage in conventional two-colortelevision systems, as, for example, in a red and green system, or in anorange-red and blue-green system. In the latter case, the orange-redcontent of the scene is reproduced in orange-red light and theblue-green content of the scene is reproduced in blue-green light.Two-color television displays, of which red-white is a special case, areparticularly well adapted for use in converting a standard monochromereceiver to a color receiver. As the result of the disclosure ofEngstrom et al. in U.S. Patent No. 2,514,043 granted July 4, 1950, thoseskilled in the art know that a standard monochrome kinescope can befitted with auxiliary apparatus by which the two primary colors of agiven system of color analysis can be caused to appear sequentially onthe viewing screen of the kinescope at the field frequency. Using theorange-red and blue-green system, for example, rotating color polarizersmounted between an observer and the monochrome kinescope and properlysynchronized with the scanning system, will cause the screen to appear,sequentially, orange-red and blue-green. In such case, the sequentialsignals that must be supplied to the control grid of the monochromereceiver in synchronism with the color of the viewing tcreen should be apair of signals corresponding to an orange-red color difference signaland a blue-green color difference signal.

The embodiment of the invention shown in FIG. 4 is also adapted to beused with a standard three-color sequential television system of thetype disclosed by V. K. Zworykin in U.S. Patent No. 2,566,713 grantedSept. 4, 1951. In such case, demodulation would occur sequentially at toprovide the red color difference signal; at 236 to provide the greencolor difference signal; and at 0 to provide the blue color differencesignal. In order to provide the proper relative gain between thedifferent color difference signals, a two-diode voltage-sensitivevariable attenuator could be used whereby the keyed matrix-amplifierwould multiply the output of the synchronous demodulator by 1.14 K/Kwhen the demodulation angle is 90; by 0.71 K/K when the demodulationangle is 236.8"; and by 2.03 K/K when the demodulation angle is 0.

Since certain changes may be made in the above method and apparatuswithout departing from the scope of the invention herein claimed, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. A television receiver for reproducing a scene being televised incolor using the red-white system of color analysis from a colortelevision signal transmitted by modulating on the main picture carrierof a television channel a composite video signal that includes aluminance signal matrixed from the red, green and blue con tent of ascanned element and representative of its brightness, a codedchrominance signal in the form of a sub' carrier of predeterminedfrequency whose amplitude and phase are functionally related to thesaturation and dominant wavelength respectively of the scanned element,and a sync signal to which the phase of said chrominance signal isreferred; said receiver comprising:

(a) means for synchronously demodulating said coded chrominance signalwith a local oscillator signal at said predetermined frequency and witha predetermined phase relative to said sync signal to obtain ademodulated chrominance signal;

(b) means including voltage-variable capacitance means for causing saidpredetermined phase to sequentially switch between two values;

(c) means to matrix said luminance signal and said demodulatedchrominance signal to obtain a pair of sequential signals, the first ofwhich is obtained when said predetermined phase has one of said twovalues and is functionally related to substantially only the red contentof said scanned element; and the second of which is obtained when saidpredetermined phase has the other of said two values and is functionallyreltaed to substantially only the green content of said scanned element;

(d) a kinescope having a viewing screen with red and minus red phosphormaterials thereon, the selective excitation of said red phosphormaterial causing said screen to emit reddish light and the simultaneousexcitation of both of said phosphor materials causing said screen toemit substantially achromatic light; and a single electron gun forproducing an electron beam focused to impinge on said screen to excitesaid phosphor materials;

13 14 (e) means to cause said beam to selectively excite one ReferencesCited or both of said materials for sequentially producing UNITED STATESPATENTS reddlsh and achromatic light on said screen; and (f) means forcausing said first of said pair of signals 2,921,118 1/1960 Belnamm 1785'4 to modulate the intensity of said beam when only 5 2955152 10/1960Keller said red phosphor material is excited, and means 3242260 3/1966Cooper et 178-44 for causing said second of said pair of sequentialROBERT L GRIFFIN Primary Emmi-"en signals to modulate the intensity ofsaid beam when 'both of said red and minus red phosphors are ex- DAVIDREDINBAUGHExami'wr' cited. 10 I. A. OBRIEN, R. MURRAY, AssistantExaminers.

1. A TELEVISION RECEIVER FOR REPRODUCING A SCEEN BEING TELEVISED INCOLOR USING THE RED-WHITE SYSTEM OF COLOR ANALYSIS FROM A COLORTELEVISION SIGNAL TRANSMITTED BY MODULATING ON THE MAIN PICTURE CARRIEROF A TELEVISION CHANNEL A COMPOSITE VIDEO SIGNAL THAT INCLUDES ALUMINANCE SIGNAL MATRIXED FROM THE RED, GREEN AND BLUE CONTENT OF ASCANNED ELEMENT AND REPRESENTATIVE OF ITS BRIGHTNESS, A CODEDCHROMINANCE SIGNAL IN THE FORM OF A SUBCARRIER OF PREDETERMINEDFREQUENCY WHOSE AMPLITUDE AND PHASE ARE FUNCTIONALLY RELATED TO THESATURATION AND DOMINANT WAVELENGTH RESPECTIVELY OF THE SCANNED ELEMENT,AND A SYNC SIGNAL TO WHICH THE PHASE OF SAID CHROMINANCE SIGNAL ISREFERRED; SAID RECEIVER COMPRISING: (A) MEANS FOR SYNCHRONOUSLYDEMODULATING SAID CODED CHROMINANCE SIGNAL WITH A LOCAL OSCILLATORSIGNAL AT SAID PREDETERMINED FREQUENCY AND WITH A PREDETERMINED PHASERELATIVE TO SAID SYNC SIGNAL TO OBTAIN A DEMODULATED CHROMINANCE SIGNAL;(B) MEANS INCLUDING VOLTAGE-VARIABLE CAPACITANCE MEANS FOR CAUSING SAIDPREDETERMINED PHASE TO SEQUENTIALLY SWITCH BETWEEN TWO VALUES; (C) MEANSTO MATRIX SAID LUMINANCE SIGNAL AND SAID DEMODULATED CHROMINANCE SIGNALTO OBTAIN A PAIR OF SEQUENTIAL SIGNALS, THE FIRST OF WHICH IS OBTAINEDWHEN SAID PREDETERMINED PHASE HAS ONE OF SAID TWO VALUES AND ISFUNCTIONALLY RELATED TO SUBSTANTIALLY ONLY THE RED CONTENT OF SAIDSCANNED ELEMENT; AND THE SECOND OF WHICH IS OBTAINED WHEN SAIDPREDETERMINED PHASE HAS THE OTHER OF SAID TWO VALUES AND IS FUNCTIONALLYRELTAED TO SUBSTANTIALLY ONLY THE GREEN CONTENT OF SAID SCANNED ELEMENT;(D) A KINESCOPE HAVING A VIEWING SCREEN WITH RED AND MINUS RED PHOSPHORMATERIALS THEREON, THE SELECTIVE EXCITATION OF SAID RED PHOSPHORMATERIAL CAUSING SAID SCREEN TO EMIT REDDISH LIGHT AND THE SIMULTANEOUSEXCITATION OF BOTH OF SAID PHOSPHOR MATERIALS CAUSING SAID SCREEN TOEMIT SUBSTANTIALLY ACHROMATIC LIGHT; AND A SINGLE ELECTRON GUN FORPRODUCING AN ELECTRON BEAM FOCUSED TO IMPINGE ON SAID SCREEN TO EXCITESAID PHOSPHOR MATERIALS; (E) MEANS TO CAUSE SAID BEAM TO SELECTIVELYEXCITE ONE OR BOTH OF SAID MATERIALS FOR SEQUENTIALLY PRODUCING REDDISHAND ACHROMATIC LIGHT ON SAID SCREEN; AND (F) MEANS FOR CAUSING SAIDFIRST OF SAID PAIR OF SIGNALS TO MODULATE THE INTENSITY OF SAID BEAMWHEN ONLY SAID RED PHOSPHOR MATERIAL IS EXCITED, AND MEANS FOR CAUSINGSAID SECOND OF SAID PAIR OF SEQUENTIAL SIGNALS TO MODULATE THE INTENSITYOF SAID BEAM WHEN BOTH OF SAID RED AND MINUS RED PHOSPHORS ARE EXCITED.